In general, as regards telephone switches such as PBXs (Private Branch eXchanges) accommodating a plurality of extension telephone sets and key telephone devices, the function of call pick-up is known. When there is an incoming call to an extension telephone set, the function of call pick-up makes it possible to give a response by deputy from another extension telephone set that is not called.
In a kind of this call pick-up function, an incoming call to an extension telephone set is picked up by dialing a special service code from another extension telephone set belonging to the same call pick-up group which is previously set. In another kind of the call pick-up function, an incoming call to an extension telephone set is picked up by dialing a special service code and a group number of a call pick-up group to which the called extension telephone set belongs, from another extension telephone set belonging to another call pick-up group. In addition, in another kind of the call pick-up function, an incoming call to an extension telephone set is picked up by dialing a special service code and an extension number of the called extension telephone set from another extension telephone set regardless of the call pick-up group. In this way, there are various kinds in the call pick-up function.
On the other hand, in a known system using a telephone switch in recent years, linking between fixed telephones and mobile telephones (hereafter referred to as “FMC linking”) such as, for example, an operation of calling a mobile telephone of an employee who is out with an extension number is conducted by using a technique of FMC (Fixed Mobile Convergence) which merges fixed telephones and mobile telephones.
In case of employees who are out of the enterprise in many hours such as employees in a business department in enterprises introducing such a telephone switch, it is conceivable that a fixed telephone is not provided on a desk of the employee and the employee uses a mobile terminal (hereafter referred to as “FMC linking terminal” for convenience) such as a mobile telephone capable of conducting FMC linking with a telephone switch not only outside the enterprise but also within the enterprise.
In a case where an employee who possesses only an FMC linking terminal picks up an incoming call in the system at that time, it is considered that the employee picks up the call from a fixed telephone of another employee because it is impossible to pick up the call from the FMC linking terminal. This is inconvenient in operation.
In connection with this, PTL 1 states that extension numbers and IP addresses of portable IP telephone sets are recorded in an extension number management table in an IP extension telephone system including portable IP (Internet Protocol) telephone sets such as mobile terminals coping with wireless LAN (Local Area Network). In addition, PTL 1 states that extension numbers are recorded in association with groups corresponding to segments of IP addresses, in this IP extension telephone system. According to this, it is said that portable IP telephone sets can be included in call pick-up groups and an incoming call in the IP extension telephone system can be picked up from a portable IP telephone set.